The use of gamma radiation cameras is well known in the prior art. Such cameras have been extensively used in the field of nuclear medicine for the purpose of producing images of specified types of tissue within the human body. Radioactive nuclides of atoms such as thallium or 99m-technetium are made part of pharmaceutical chemicals which attach themselves to specified human tissues, such as bone tissue, muscle tissue, or blood tissue. Once the radionuclide-labeled chemical has been introduced into the body, and has had time to attach itself to the tissue to which it has a specific attraction, a gamma camera can be used to form an image of the resulting distribution of radioactivity. An important advantage of this diagnostic technique is that it permits noninvasive investigation of a variety of conditions of medical interest. For example, by injecting into a patient a radioactively tagged chemical which attaches to muscle tissue and by forming an image of the resulting radiation emitted from the various parts of a patient's heart, it is possible to determine which parts of that heart are receiving blood, as is required for heart tissue to remain healthy.
The gamma radiation camera was first developed by H. O. Anger in the late 1950's. His camera uses a large sodium iodide (NaI) scintillator crystal which emits visible light when struck by gamma ray or X-ray photons. A pinhole or a parallel hole collimator is placed between the scintillator crystal and the source of radiation to be imaged. On the opposite side of the scintillator crystal a matrix of photo-multiplier tubes is spaced a slight distance from the crystal, so that the relative amounts of light detected by the various photo-multiplier tubes enables a determination of the location within the crystal at which a scintillation resulting from the collision of a gamma ray or X-ray occurs. The pinhole causes the image of the radiation source to be focused on the surface of the scintillator crystal in a manner analogous to the operation of a pinhole camera. When a parallel hole collimator is used with such a scintillation crystal, most of the photons which reach the crystal are traveling generally perpendicularly to its surface and thus there is a correlation between the location at which such photons hit the scintillator and the position from which they are emitted.
Although the Anger camera represented a great advance in the field of nuclear medicine, it has several disadvantages. A first is that its energy resolution is normally limited to approximately ten to fifteen percent, which prevents it from distinguishing properly between primary photons emitted directly by a radioactive nuclide, which have the full energy of photons associated with that nuclide, and secondary photons emitted as a result of the Compton scattering of such primary photons. Such secondary photons have less energy than primary photons because of the energy lost in Compton scattering. Radiation images produced by cameras which can not distinguish between primary and secondary photons indicate many points as being sources of primary radiation which are only points at which primary photons have undergone Compton scattering, and thus such images are undesirably blurred.
Another problem associated with Anger type gamma cameras is that their use of pinholes or parallel hole collimators greatly reduces the number of photons emitted by radioactive sources which are able to hit their scintillators. As a result it is necessary either to use large dosages of radioactive chemicals or long exposure times in order for a sufficient number of photons to be counted by the camera to produce a proper image. A further problem associated with such cameras is their low resolution. Even the best Anger cameras have a resolution of one to one and one half centimeters at ten to twelve centimeters from their pinhole or parallel hole collimator.
A gamma camera has been developed which uses a solid-state detector in conjunction with a parallel hole collimator. Such a camera is disclosed in U.S. Pat. Nos. 4,047,037 issued to Schlosser et al. and 4,055,765 issued to Gerber et al. This camera, instead of using a sodium iodide scintillator crystal, uses a solid-state detector made of the semiconductor germanium. When a photon hits an atom of germanium in such a detector, the collision creates electron-hole pairs. The germanium crystals are relatively flat, and on one side they have one or more electrodes for attracting the electrons and on the other side they have one or more electrodes for attracting the holes so produced. The electrodes on one side of the crystal determine the location of that collision along one axis and the electrodes on the opposite side determine the location of that collision along a perpendicular axis. The device also includes circuitry for determining the amount of electrons or holes that are released in any given collision. As a result, the device is capable of indicating an x position, a y position, and an energy associated with each photon collision which takes place with its semiconducting crystal. This semiconducting detector is used in conjunction with a parallel hole collimator, so that there is a correspondence between the location at which a photon hits the detector and the location from which that photon has been emitted.
Solid-state gamma cameras of the type disclosed in the Schlosser et al. and Gerber et al. patents have a much higher degree of energy resolution than is possible with gamma cameras using sodium iodide detectors, enabling the rejection of most unwanted secondary photons. This is true because it takes only approximately 2.9 electron volts (ev) for an electron-hole pair to be created in a collision with germanium, whereas it takes approximately 300 ev, or one hundred times as much energy, to create a photon of visible light in a collision with a NaI scintillation crystal.
Unfortunately the use of parallel hole collimators in the gamma cameras disclosed in the Schlosser et al. and Gerber et al. patents lets only a very small percent of the photons emitted by radiation sources through to their detectors, and thus either high radiation doses or long exposure times are required for use with such gamma cameras. In order to reduce the negative impact of their parallel hole collimators o the number of photons which reach their detectors to a reasonable level, gamma cameras of the type disclosed in the Schlosser et al. and Gerber et al. patents normally use collimators which are no more than two and one half centimeters thick. As a result, the resolution of such cameras at a distance of ten to twelve centimeters is usually worse than one centimeter.
A gamma camera which operates on a somewhat different principle was described in Session 2B-3 of the 1981 IEEE Nuclear Science Symposium, which took place at the Sheraton-Palace Hotel, in San Francisco, Calif., on Oct. 21st through 23rd, 1981. This session, entitled "Comparison of Reconstruction Algorithms for an Electronically Collimated Gamma Camera" was authored by D. Doria and M. Singh of the University of Southern California. The electronically collimated gamma camera referred to in that session has a flat sodium iodide scintillation crystal and associated photomultiplier tubes, like an Anger camera, together with a solid state detector of the type described in the Schlosser et al. and Gerber et al. patents, referred to above. The flat solid-state detector is placed parallel to the scintillation crystal and between that crystal and the source of radiation. If a photon undergoes Compton scattering with the solid-state detector and then collides with the camera's scintillator, the solid-state detector indicates the energy and location of the Compton scattering and the scintillator indicates the location of the scintillation collision. From this information and a knowledge of the expected energy of the gamma rays emitted from the source of radiation, the directional line which the photon travels between its collisions in the solid-state detector and in the sodium iodide crystal and the angle of Compton scattering at the collision in the solid-state detector can both be calculated, defining a cone of possible paths of the gamma ray before its collision with the solid-state detector. Such a cone has the location of Compton scattering in the solid-state detector as its tip, the translation line between the locations of the Compton scattering and the scintillation as its axis, and the angle of Compton scattering as its opening angle, i.e. the acute angle between the axis and wall of the cone.
Unfortunately, because the electronically collimated gamma camera only determines a possible cone from which each gamma ray it detects could have come, it requires the use of tomographic techniques, such as those described in the Session 2B-3, mentioned above, to generate a two dimensional image. Such tomographic techniques, which attempt to locate the origin of photons by finding the intersections of the cones of possible paths calculated from different collisions, require considerable computation and a great number of photons to be detected in order to give an image of reasonable quality. In addition, although the solid-state detector of the electronically collimated gamma camera provides relatively good energy resolution, the scintillation crystal used in such a device does not. For this reason the electronically collimated gamma camera does not have the ability to accurately distinguish between primary photons emitted directly by the source of radiation to be imaged and unwanted secondary photons emitted after Compton scattering.